Semiconductor manufacturers thoroughly test some integrated circuits before they are shipped to a customer. Testing may screen for basic functionality as well as compliance with operational specifications driven by customer requirements. SRAMs are an integral part of many semiconductor devices, and are included in the functional tests. An SRAM is composed of many bitcells arranged in blocks optimized for compactness and modular placement on the device. In turn, each bitcell contains a number of transistors.
With transistor and gate length scaling to 65 nm/45 nm nodes, ensuring SRAM stability over the product's lifetime has become increasingly difficult. Negative bias temperature instability (NBTI), defects, or other phenomena may be manifested as a transistor threshold voltage (Vt) change over time. Such changes may cause the minimum reliable operating voltage (Vmin) of SRAM memory cells to drift, or to become too stable to change state when commanded, leading to device failure.
Manufacturers typically screen such failures by a burn-in procedure, in which the device is operated under conditions that accelerate physical mechanisms associated with the failure. When an SRAM fails during burn-in, the failure lowers product yield. Yield loss may result in higher costs to the manufacturer, but protects the customer from the failure. But the burn-in may fail to sufficiently accelerate some failure modes, which may lead to premature SRAM failure after installation by a customer. In these cases, the manufacturer risks liability and loss of customer good-will.